The present invention relates to switched-capacitor filter circuits and in particular to such a circuit having at least one simulated inductance and having a number of controlled switches and capacitors as well as an operational amplifier.
Switched-capacitor (SC) filters are known from the article "Switched-Capacitor Filter Design Using the Bilinear z-Transform" in the periodical "IEEE Transactions on Circuits and Systems", Vol. Cas-25, No. 12. December, 1978, pages 1039 through 1044 as well as from the article "Switched-Capacitor Circuits Bilinearly Equivalent to Floating Inductor or F.D.N.R." in the periodical "Electronics Letters", Feb. 1, 1979, Vol. 15, No. 3, pages 87 and 88. Such filters do not process time-continuous analog signals in the true sense, but rather process time-discrete signals which exist in the form of samples, which samples are generated according to a sampling frequency F by the relationship T=1/F, where T is the sampling period. Circuits for generating such samples are known to those skilled in the art and are not explained in detail herein. Such sampling circuits may be pre- or post-connected to the known circuits illustrated, so that samples derived from an analog signal are supplied to the filter circuit at its input side and the signals available at the output side are re-converted into time-continuous analog signals. The significant technical advantage of such filters is that coils are simulated by means of active circuit elements and capacitors, which are suitable for the monolithic integration of larger filter circuits. Conventional operational amplifiers are predominantly employed as the amplifiers in those circuits and accordingly design objectives are to achieve the smallest possible number of circuit elements while still insuring the stability of such circuits.
In the aforementioned known circuits, the inverting input of the amplifier is connected via a capacitor to the output of the amplifier, a type of connection sometimes referred to as "counter-coupling" or feedback connection. Some operational amplifiers employed are at times not counter-coupled or may require a high common mode rejection, because the inverting input of the operational amplifier is briefly not counter-coupled during certain switching phases, or the non-inverting input of the amplifier is not always held at ground potential. Moreover, the realization of the capacitors in such circuits is generally undertaken by metal-oxide-semiconductor (MOS) technology and the unavoidable ground capacitance associated with every floating MOS capacitor can lead to significant disruptions of the filter function.
A switched-capacitor filter circuit having at least one simulated inductor which can be employed both as a floating as well as a single-sided grounded coil and in which disruptions occurring due to switching operations are minimized is disclosed and claimed in my co-pending U.S. application Ser. No. 173,759, filed July 30, 1980 corresponding to German application P 29 32 419.4, filed in Germany on Aug. 9, 1979.